The amino acid sequence for the non-thrombin portion of the human prothrombin molecule is to be determined. The properties of human prothrombin derivatives formed by digestion with thrombin are to be determined, and compared with those obtained from the bovine species. Attempts are to be made to contribute to our understanding of the structure of prothrombin by employing calculations based on the amino acid sequence, and in the case of thrombin, by X-ray diffraction of crystals. Autoprothrombin II-A is an inhibitor of blood coagulation. It is derived from Protein C by activation of thrombin. This process is to be studied. Finally, the changes produced in purified Ac-globulin are to be studied when the molecule is digested with purified autoprothrombin C. The degradation products are to be characterized with respect to physical properties and biological activity. One of the subunits is to be studied with respect to amino acid sequence. A review on snake venoms and blood coagulation is to be completed. BIBLIOGRAPHIC REFERENCES; Human and Bovine Prothrombin Similarities. Daniel A. Walz, Walter H. Seegers, Houria I. Hassouna and Jan Reuterby. Thrombosis Res. 4: 875-878 (1974). Prothrombin and Thrombin: Selected Aspects of Thrombin Formation, Properties, Inhibition, and Immunology. Walter H. Seegers, Houria I. Hassouna, David Hewett-Emmett, Daniel A. Walz, and Thomas J. Andary. Seminars in Thrombosis and Hemostasis 1: 211-283 (1975).